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How does skydiving flight cause shock cooling?
While answering this question (How can reducing power too quickly on a piston engine damage it?), barit1 said that:

Skydiving planes see a LOT of shock cooling, and they pay for it at overhaul time

What exactly happens during a skydiving flight which causes a lot of shock cooling?

Skydiving flights are abusive on engines for a number of reasons, but it's all tied to the fact that these flights are all about getting up to altitude, kicking the jumpers out, and putting the plane back on the ground as fast as possible so you can pick up the next group and do it all again, as many times as possible in a day.

While all skydiving descents aren't as dramatic as the video I linked to its not uncommon for skydiving pilots to make a very rapid descent at or near idle power. This causes a more drastic temperature change in the engine (particularly the cylinders, which are not producing as much heat but still have lots of air blowing through the fins).
In addition to the potential damage from rapid temperature changes the engine is also spending relatively little time at cruise power (it's either at full throttle climbing or near idle descending), and depending on the operation may be shut down between jumps. This is far from the ideal situation where the engine would run at 60-75% power as long as possible.

Just curious, are the acrobatics the pilot used in that video safe for the airframe?

@flyingfisch There is quite some discussion of that in the Youtube comments - short answer: "Well, the wings didn't fall off!" (Longer answer: I think everything demonstrated can be done as a 1-G maneuver, but I've no idea what the plane they used is certificated for)

that video has been taken down.

Because they tend to climb quickly to get to jump altitude (high power, hot engine) then descend very quickly to pick up the next batch of sky divers.  Time = money.
Close the throttle, point the nose at the landing site.
However paul indicates in the comments that he worked at a centre where they took this into account and exercised caution (good airmanship) in how the engine was treated.

That's only half of it.  Before the descent, the jump plane will make a full throttle climb to altitude.  In other words, it heats the jugs up as much as it can during the climb, gets up to where the air is cold, and then dives throttle closed back to lower altitude.  The dive starts up where the temperature is very low.

the PC-6's claim to fame was "10 jumpers to 10K in 10 minutes"

The aircraft in the referenced video showing a very rapid descent is a King Air (or similar). Turbine aircraft really don't have shock cooling issues - standard procedure is engines to flight idle, props to flat pitch, nose down to Vne. Planes like a Pilatus Porter can descend faster than freefalling jumpers and are often used as extras in films (you need to plan these shots with the pilot).
"Shock cooling" by definition is a temperature change faster than the metal can react, leaving you with a cylinder that is hot on one side and cool on the other. As I mentioned elsewhere, this is caused mainly by bad airmanship. My place covered this quite well in training, and we didn't have too many problems.
(pop quiz moved to separate question )

Putting to the props in before the descent is a great way to upset the neighbors.  Most places I've jumped at, the pilots seem to leave the props at the climb setting until in the pattern.  The noise difference in the descent is dramatic, and our chief pilot says it doesn't make a huge difference in his turn times.  Since every airport seems to be surrounded by NIMBYs these days, it makes sense to try to play nice.

Everywhere I've jumped didn't have neighbors.

Shock cooling is not a problem in reciprocating aircraft engines.  In fact, today you almost only hear it mentioned in the context of turbocharged aircraft, and even then to avoid shock cooling of the turbocharger.  Another common misconception.
You'll find no mention of shock cooling in the Pilot's Operating Handbook.
http://www.avweb.com/news/pelican/182107-1.html?redirected=1

While I generally agree with John Deakin (and other smart folks who say shock cooling isn't a big concern), you will find it mentioned in Lycoming SI 1094D - Presumably they would not have taken the trouble to specifically call it out in this service instruction if it were completely mythical, but their limit of 50°F. per minute cooling rate is pretty generous for most operations.

Shock cooling of Turbochargers is an old wife's tale? Excuse me while I giggle.

Also, this wasn't a question about whether shock cooling exists or not, but about what parts of this particular flight activity would exacerbate the (potential) issue.

If you run the cylinders to redline and then, without having a chance to cool to normal operating temp (~380°F max), pull ghem to idle and dive back down to the airport it is conceivable that shock cooling might be a problem.  THAT is poor technique.  Most skydiving engines that I've seen run past TBO and many with the original cylinders!

Shock cooling is a myth.  Or at least in these kinds of applications.  Multi engine trainers like the Piper Seminole have their critical engines routinely shut down during training flights with no damage as a result of shock cooling or reductions in TBO between the critical and non-critical engines.  Maybe if you ran the engine at 85% power on a load cell for an hour to heat soak it, then quickly dunked the crankcase in a vat of ice water could you damage an engine this way, but not with air cooling.

Also shock cooling of a turbocharger is highly unlikely as well.  You do have to run it at idle power settings for approx 5 mins aster landing to reduce the temperature in the hot section to prevent oil from boiling in the bearings after shutdown.  But that is a different matter.

Having flown skydiving ops in both a C182 and a C208, I can make a few comments.  First in the C182, the practice was to descend with cowl flaps closed, the throttle retarded but not at idle.  "Clearing" the engine was done, and typically I would use a 50 degree (plus or minus) turn to maintain situational awareness and to increase the induced drag, and descend quickly.
In the C-208 the operation was similar, except that power was brought back, and there was no clearing of the engine, as it is meaningless with a turboprop.
On both aircraft I would frequently descend with full flaps. I did not use Vne descents, as often the air was not smooth, and the full flap descent at a lower speed offers adequate visibility and a steeper descent than just Vne.
The C182 was not turbocharged, and I know of no one in the area who uses a turbocharged recip for skydiving, although I am sure there are people who do elsewhered
Also, in the C182 normally there was a few minutes of flight at reduced power (75 or 70%) while ATC was contacted and the jumpmaster got happy with the traffic in the area, and where he was WRT the dropzone. This allowed CHT to drop more gradually.


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How much jurisdiction does the FAA have over military aircraft?
The FAA controls just about everything about civilian aircraft in US airspace in terms of regulation, certification, flight rules, etc. Do they have any authority over military aircraft beyond requiring military flights to communicate with ATC for safety and designating certain areas as military airspace?
This question addresses retired military aircraft in civilian use, and one of the answers to this question mentions a "Surplus military" category, but nothing about active military.  I'm asking about current, active military aircraft in terms of certification, pilot licensing/qualification, etc.

If you include as military aircraft civilian operators in civilian aircraft flying military contract flights, those flights are subject to all the usual FAA regulations.

If they are being operated by the military, not much. Military operations can exempt themselves from the FARs (as a matter of practice they don't: When operating in the US National Airspace System they follow the same operational regulations we do, but as a matter of regulation they're subject to the military's rules for airworthiness, maintenance, etc.).
Military pilots have a military pilot's license (which can be converted to an FAA license with a little paperwork), and the airworthiness and maintenance standards for active military aircraft are defined and managed by the branch of the service responsible for the aircraft. (This is similar to, but legislatively distinct from, Public Aircraft Operations.)
This doesn't mean the FAA is completely silent about military operations/aircraft: There is guidance for military aircraft that are based on commercial designs (also available in an easier-to-read Advisory Circular: AC 20-169), which would be applicable to aircraft like the VC-25 ("Air Force One").

The US military doesn't actually issue a "military pilot's license".  Instead, the authorization to fly as a pilot is the cumulative effect of multiple other authorizations, the most famous of which is the pilot/aviator badge worn on a military uniform.  The pilot badge in the Air Force represents a Pilot rating in the Air Aorce. There are other non-pilot ratings in the Air Force as well, besides pilot.

As mentioned here and here, FAA's responsibilities include:

Developing and operating a system of air traffic control and
  navigation for both civil and military aircraft

However, FAA does not govern military aircraft. The military has their own rules and regulations, but the military follows FAA regulations when flying in National Airspace. There is airspace in the US and elsewhere that is set aside for military operations such as the Barry Goldwater Gunnery Range. Military jets can fire at targets on the ground and civilian aircraft are kept out. There are many others.
As needed, FAA does make special allowances for the military. Below is an example of a Military operations area.

One minor nitpick: The FAA does not "prohibit" non-military aircraft from entering MOAs. This is a common misconception (and perhaps one the military would like to see become reality), but it's not the case. IFR aircraft will be routed around active MOAs if ATC cannot ensure IFR separation requirements will be met, but it is perfectly legal to fly VFR through an active MOA. It is however prudent to avoid doing so where possible, or at least exercise caution if you must. (Ref. AIM 3-4-5

Yeah, Voretaq just beat me to it. You can enter an MOA, even if it's not necessarily a good idea. MOAs are supposed to be used for non-hazardous military activity. Actual restricted airspace is used for areas where the military is performing potentially hazardous activities.

As noted above, the Barry Goldwater Gunnery range is in R-2301 E, a Restricted area.

@NathanG I would hope so if they're firing at the ground. - lol - I'm thinking that an MOA like the ones in the image that end at 500 ft AGL wouldn't work well for such operations.

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How are squawk codes assigned?
How are the 4-digit squawk codes assigned to individual flights?
This answer indicates that there should be a "correct" one. How does ATC know what is "correct" and what is "incorrect"?

To start out with, I'll be referring to how the US does it, and it's generally laid out in the National Beacon Code Allocation Plan. Every facility has various blocks of codes that they can use to assign to flight plans, as described in the linked document. For example, a smaller tower/approach control, could use the codes 0101-0177 for IFR flights that are only in their airspace, and 0201-0277 for the VFR flights that stay in their airspace, and a Center would have a similar bank of codes for flights just in their airspace, and then a different bank for codes that go to different centers. The whole purpose to these allocations, is to minimize the need to change the squawk code while in the air, and to avoid overlap with adjacent facilities.
As flight plans are submitted to the various computers, about 30 min before an Instrument flight plan's proposed time, it's officially assigned a code from the available blocks, and it's printed at the departure airport or facility that'll first work it. Flight plans stay in the system, depending on however long the center sets. Often it's 2 hours, sometimes 3, but during bad whether when lots of flights are being delayed, it could be extended to 4+ hours. The drop interval helps to keep enough codes available for aircraft intending to depart, while allowing flight plans to go away if something happens and the flight won't be departing.
If the aircraft just randomly calls ATC, the controller types it in to either the radar or flight plan system, depending on what extent the pilot wants services, and then the system prints out an appropriate beacon code.
To answer "How the controller knows whether the code is correct or not:" If the pilot squawks the correct code (i.e. the code that the system has associated with the flight plan data for that flight) then a "data block" appears on the scope that is correlated with the radar target. The data block contains useful information: A/C ID, A/C type, Mode C altitude, radar-calculated groundspeed, and perhaps other information. If the pilot squawks an incorrect code the system will not correlate the proper data block with the radar target. Then the controller knows the pilot is not squawking the assigned code.

We also have the option of manually assigning a squawk code when entering the flight plan. Generally you don't want to do this when entering someone for NAS processing because you have no idea whether a code is in use, but when a bunch of VFR guys call at the same time and you aren't able to enter them all in, you can assign codes from your facility's private bank (as you describe) and type them in later as they're taxiing out.

And, this is why my question should have been closed as "too broad". @randomhead, this really shouldn't have been edited into someone else's answer, you should have written your own to answer this part of the question. Of course, that should have been removed from the original question and asked on its own.

A squawk code (aka Mode A code) is used in Flight Data Processing systems to correlate a radar track (or other surveillance system track) to a flight plan. Basically the radar downlinks the squawk code from the aircraft and the FDPS looks up the associated flight plan. Usually there are a number of codes which do not link to a flight plan but are reserved for local purposes and VFR flights without a flight plan.

In Europe there are 3 Mode A code allocation systems in use:

Originating Region Code Assignment Method (ORCAM)

Under ORCAM aircraft are assigned a code on departure and will maintain this code until they fly into a region where their code conflicts with another flight. Then it is reassigned. For every region there are 'local' codes which are only used for regional / domestic flights, and there are 'transition' codes for region boundary crossing flights.

Centralised Code Assignment & Management System (CCAMS)

In CCAMS a computer system allocates codes to flights based on the planned trajectory through the CCAMS area and ensures there will never be a code conflict.

Elementary Surveillance (ELS)

When using ELS, if a flight needs to be (re-)assigned a mode A code and it is in Mode S radar coverage and the downlinked aircraft ID correlates with the filed flight ID on the flight plan, Mode A 1000 is assigned. Squawk 1000 serves as a signal to the Flight Data Processing System to correlate the radar track to the flight plan based on the downlinked Mode S aircraft ID instead of the Mode A code. This method is usually used in combination with ORCAM, sometimes with CCAMS. By using ELS, multiple aircraft fly with squawk 1000 which frees up other Mode A codes.
This Eurocontrol document (PDF) gives some more detail about the code allocation schemes in use.

South America and the Caribean use the ORCAM (PDF) system.

In the US, the codes follow section 5 of the 7110.65. Non-discrete codes end in 00 and can cover multiple aircraft. Discrete codes never end in 0 and are specific to one aircraft.
"How does ATC know what is "correct" and what is "incorrect"?"
The computer system generates them. There are certain "slot ranges" for every facility. It's not that the controller punches in something he came up with on his own. He requests a squawk from the system, and then the computer spits out one, and that's the one he/she tells the pilot over the radio. To answer your question on how it knows correct vs incorrect, I'd have to say that this is implemented correctly in the software that runs the backend.

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Are NACA airfoils used in modern aircraft design?
I know that NACA is famous for its ducts:

Gratuitous Ferrari F-40 from Wikipedia. Mmmm... pretty...
It is also well known for its airfoils, and many planes in the early years of aviation used NACA designed airfoils for their wings (for example the Piper Cherokee referenced here).

Piper PA-28 also courtesy of Wikipedia
Do current aircraft manufacturers still use stock NACA airfoil designs, start with a NACA standard design then modify it to meet needs, or are the airfoils completely bespoke?
I'm asking about the major airliner manufacturers like Boeing & Airbus, regional jet builders like Embraer, as well as private/GA companies like Cessna, Gulfstream, and, of course Piper. Information on the (few, dozens, hundreds?) of other aircraft manufacturers (and kit designers) that don't pop to the top of my head is also welcome and appreciated.

Thinking about this further, I realize it could quickly devolve into a very broad this aircraft uses a NACA standard wing, that one doesn't list. That's not really my intent. A simple yes, this aircraft/wing from 5-10 years ago uses a NACA standard would answer the question completely, as would a the last one I'm aware of is from 35+ years ago.
Also, the use of "modern" is rather vague as the Boeing 757 certainly feels "modern" (having been launched in 1982) but has been out of production since 2004, while the 747 started production in 1969 and is still being built. If someone wants to jump in to help define "modern", I'd take no offense.

The Savannah ultralight uses a NACA airfoil, for what it is worth.

According to Wikipedia, it started production around 2000, so that would certainly be "modern", and the "Airfoil: NACA 65-018 (modified)"

Just three data points: The tail surfaces of the Pilatus PC-12 still use the venerable NACA 0012, even though a better alternative (from the Wortmann FX 71 L series) was proposed. It did not help that the Wortmann airfoil is used on many small airplanes, has more lift and less drag and an abundance of data exists on it: The (mainly British) engineers at Pilatus were too conservative to use anything newer than an airfoil from the 1920s.

Pilatus PC-12 (picture source).
The wing of the Dornier Seastar, designed by a group of old Dornier engineers in the early 1980s, used the same airfoils as the venerable Do-17 of the 1930s, namely the NACA 23012. Later flight tests showed the need to add a leading edge droop on the outer wing, a modification that cost several knots in top speed and would have been entirely avoidable with better airfoils and/or the use of washout.

Dornier Seastar. Picture by Rschider (own work).
The excellent Polish aerobatic glider Swift S-1 uses indeed a NACA 6-digit airfoil, the $64_1412$. As Jerzy Makula explained to me, it gives excellent snap roll and spin control, and when flying the prototype, I could control the exit direction from a spin within ±15° with very little practice. Jerzy simply took what had worked before.

Swift S-1. Picture by TSRL (own work).
Generally, small companies or experimental airplane builders still rely sometimes on the NACA series, but in general better alternatives exist. Big companies use custom-made airfoils, helped by computer codes which model not only the two-dimensional flow around the isolated airfoil, but can optimize the specific wing section by including the influence of engine nacelles, pylons and the fuselage. Well-designed wing roots do not use one single airfoil, but a three-dimensional shape which minimizes interference drag.
This is true even for glider companies. While Wolf Lemke used his own creations for the early LS series of gliders on the basis of their good looks (and with good success), todays gliders are designed in the computer, helped by university departments which relish the chance to let students work on real-world designs.

This answer gets the checkmark nod because the planes mentioned are the newest of the bunch listed in all the answers (so far). I'm not sure I'd call 1991 "Modern" in 2015, but that may well be about as modern as GA aircraft get...

@FreeMan: I agree - modern is relative here. The Swift is modern in that it uses glass fiber, but so does the Seastar. However, in my opinion, the Seastar is really old-fashioned, because its designers learned their trade in the Thirties and seem to have never improved their knowledge later in life. The hull is really a result of studies from the Twenties, when heavy, low-powered engines needed the least amount of water resistance to get the craft to take off, at the cost of much lower ride comfort. The same goes for the wing, old-fashioned and well below its potential, obviously.

NACA airfoils are used still used in aircraft, though most of the airfoils used are modified in some way.
For example, the Cessa uses either pure or modified NACA airfoils in most of its Cessna Citation jets, though newer models tend to use Cessna's own airfoils.
For example, the Citation V uses modified NACA 23014 airfoil at the root and modified NACA 23012 at the tip.

"Cessna uc-35a citation 560 ultra v arp" by Adrian Pingstone - Own work. Licensed under Public Domain via Commons.
The recent ones, like the 750 Citation X uses Cessna's own Cessna 7500 airfoil.
Boeing, on the other hand has been mostly using its own airfoil designs from day one, though it has 'borrowed' ideas from NACA series (like supercritical design)
On the military side, the F22 Raptor uses a modified NACA 6 series airfoil. According to Public Domain Aeronautical Software

F-22 uses a uncambered 64A section with 5.92 percent thickness at the root and 4.29 percent thickness at the tip

For a detailed list please see The Incomplete Guide to Airfoil Usage.

Just noticed that you quote the same NACA 23014 airfoil at the root and wingtip of the Citation V. Is it the same airfoil across the entire wing with different modifications at each end, or is there a typo in one number or the other?

@FreeMan It was a typo. Corrected. Thanks for noticing.

The Cessna 172R uses a modified NACA 2412 airfoil and has been in production since 1956 to the present. This also substantiates the excellent points raised by aeroalias (https://en.wikipedia.org/wiki/Cessna_172).
This airfoil guide also specifies the airfoils used for wing root and wing tip for a large range of aircraft and helicopters (http://m-selig.ae.illinois.edu/ads/aircraft.html).
From this airfoil guide, you can also check out any "modern" aircraft you are interested in and determine if the NACA airfoils or any other type of airfoils are used.
P.S. I just saw that aeroalias also referenced the airfoil guide. Do check it out, it's a fantastic resource.

The 172R started production in 1996 and ended production soon after when the 172S was debuted. Your dates are correct for the Cessna 172 as a whole, though 172 production stopped for a decade between 1986 and 1996.

There is theory, and then there is real life. The advantages (for general aviation) of modern airfoils fall mostly in the former category.
This paper on airfoil selection explains this much better than I ever could. My designs (full disclosure: these exist only on paper up to now) tend to get a "catalogue" airfoil, because the properties and behaviour can be predicted beforehand.
I would hate to design an aircraft, then put in 4-5 years of work building the thing and finally end up with an unmanageable beast because I changed one parameter of a bespoke airfoil in the wrong direction.
To prevent that from happening I can live with almost any (reasonable) reduction of theoretical efficiency.


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How do flares work?
How do fighter jet flares work?
What is the concept behind them and how do they stop impending missiles during flight?
And also how do pilots know when a good time to launch flares is?

You have 3 questions here (Or maybe 2 because the first and second are very similar)
1 and 2:
Flares are some sort of burning compound that generates a significant amount of heat to distract (hopefully) an Infra-Red guided missile. They attract the IR missiles because the amount of IR energy created by them may be greater than that of the exhaust, which is what IR missiles follow.
3
The flares are deployed when the plane detects the presence of an IR missile. (Wikipedia)

Once the presence of a "live" IR missile is indicated, flares are released by the aircraft in an attempt to decoy the missile; some systems are automatic, while others require manual jettisoning of the flares.

They also deploy them for show too.

Flares use a charge of magnesium or other substance which burns very hot and emits a very bright source of infra red energy, the theory being that an IR guided air to air missile will acquire this 'spotlight' of infrared energy and track it as opposed to continually tracking the IR source produced by the defensive aircraft's jet engines and exhaust plume.
Flare dispensers, such as the SUU-25 common countermeasures flare dispenser systems are small boxes mounted flush with the aircraft skin in various places on the fuelage of wings of a jet.  Each box is an array of tubes, each loaded with a single MJU-7A/B decoy flare.  A schematic of the flare is shown below.

From Wikipedia:

1); an electrical impulse cartridge (2), providing both expulsion and,
  in some cases, direct ignition of the payload; a pusher plate acting
  as a safe & arm device (3); the payload (4) with first fire layer (5);
  the wrapping self-adhesive polyester reinforced aluminum foil (6); and
  a front washer (7).

You've only answered part of the question here.


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Does the Piper Aztec have flaps and slats and if so what kind?
I was just wondering if any one knows if the piper Aztec has flaps and what kind. Also if the piper Aztec has any other high lift devices.

The Piper Aztec has standard (plain) flaps:

Source: Sky4Buy.com
Another View:

Source: Same as above
The Aztec does not have slats as can be seen in the image from the front of the wing:

Source: NWAviators.com
Here is a close-up view of the wing of the plane in the first two images (damaged, but no slats):

Source: Sky4Buy.com
Other than the flaps (and wings) there are no other high-lift devices installed on a Piper Aztec.


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How far away from a wing's surface does the air stream remain unaffected?
I would like to know how far above and below a wing's surface an observer must look in order to see no change to the airflow. Not simply visually but mechanically, assuming a perfect scenario. I am not particularly adept at mathematics, so if there is a rule of thumb, or a general upper boundary, that would suffice for the figure I need.
For a visual representation, a wind tunnel with smoke trails. At some point distant from the top and bottom of the wing in the tunnel, all flow lines will be identical again, implying that wings have an area of effect. How far is this point from each respective surface?
Another way to ask would be to say how far apart must biplane wings be (in a perfect scenario) for them to have no effect on each other?

I don't know the answer, but I will assume that the person who does will need to differentiate between a discussion about boundary layers and wake.  Just like a boat in water, the distance away that water flow is affected by the passing hull depends on the time that has passed since the boat disturbed the point that is lateral to that wake as it moves outward.  For biplane wings, I would guess no more than two feet would probably do the trick.

I guess ground effect could be used, at least as a rule of thumb... i.e. if the aircraft is within a certain distance of the ground, an increased lift effect becomes noticeable. (Due to changes in the flow patterns around the wing.)

A precise answer would be: Only outside of the supersonic shock cone. In subsonic flow the change has unlimited range, but dies down with the inverse of the distance.
A mathematical description is possible with the Biot-Savart law. It was first formulated for calculating the magnetic field induced by a current, but can equally well be applied to aerodynamics.
Now it depends on how much error you still accept. If you need the interference-free distance between two biplane wings, express that distance in multiples of the wingspan - that is how it scales. A few wingspans of vertical separation should be enough to make the interference between wings negligible. In flow direction the distance must be much larger: Munk's displacement law postulates that in inviscid aerodynamics the distance in flow direction between two wings makes no difference. In reality, air traffic control requires a separation of up to 8 nautical miles between heavy airplanes for the turbulence to die down or be washed away by wind.

Ok then let's elaborate slightly. If I have a wind tunnel and I place a wing inside, how far away from the sides of the tunnel would the wing have to be for the pressures on the tunnel caused by the wing to be negligable? Ten wing thicknesses? Twenty? Exactly five metres?
Please forgive my naivety and lack of mathematical prowess.

What I need is to be able to say something like:
"In normal air at sea level, for a wing of 3cm thickness, the area of effect would be about 10 cm above and below" .. sort of thing, This is a model so it does not need to be precise.

Any other wing inside of those 10cm would therefore experience lifting issues, and any outside would experience little to no interference. Obviously it isn't 10cm, it would be some function of the air density and distance; the scale of the wing compared to the tunnel environment so to say.
If the wing is 'this big', how big does the tunnel have to be to satisfy negligible interference. It must be a fairly standard thing?

@Stephanie-mayShellam: Windtunnels use correction factors to compensate for the tunnel wall influence. Windtunnels are too small to ever have a distance to the wall where no wall influence remains.

What if the wind tunnel was a 1000mm across and the wing was 1mm across? Would interference still occur at a measurable level?
Is there a ratio between wing size and its measurable effect at distance, and if so what is a rough guide? This is all I want to know. Reasons are irrelevant, just pure theory.

@Stephanie-mayShellam A 1mm wing in a 1000mm wind tunnel would cause negligible interference. While theory would still show a minute difference, it would be unmeasurable. However, a 1mm model is completely impractical and would be dwarfed by the sting it needs to ride on. The sting would still cause measurable interference.

2 cents here: One of the first homework assignments of our aerodynamics class was to calculate the lift of an airfoil by using the pressure data of the wind tunnel walls. Any wing shape would generate a pressure variation (if it's generating lift) at the boundaries of the tunnel. (Unmeasurable it may be)


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Do ballonets control the buoyancy of a Zeppelin NT?
Ballonets are bags of air inside the lifting body of the airship.
I have read about Zeppelin NT, a semi rigid helium filled airship. It uses ballonets to maintain constant over-pressure inside the aerostat. I can't see how this airship would use ballonets to ascend/descend. However it has swiveling engines and actually takes off heavier than air.
I have read that ballonets were also used to control altitude of an airship. However I can't see how it could work since air is neutrally buoyant in air. Maybe in older airships the air in the ballonets was slightly pressurized to make them heavier. Maybe on the landing hydrogen was released and ballonets were filled with air to maintain constant pressure.

The NT, being semi rigid, does not work like a balloon where buoyancy of the airship is controlled by adding or releasing helium. The quantity of helium carried is set at the beginning of the flight, and is set so that the aircraft is heavier than air. Vertical motion is then controlled by engine power. Buoyancy control is not the ballonet's purpose.
Ballonets are used to maintain constant pressure in the hull, as this is required for structural integrity. Whatever additional volume is required beyond that consumed by the helium carried is filled by air pumped into the ballonets. This air may also be released to control pressure as the craft ascends, and pumped back in as the craft descends. The hull is not designed to be pressurized beyond 5mbar.
Zeppelin GMBH technology

Envelope - Non-flammable Helium, contained in a high strength, multi-layered laminate, provides the necessary lift. The envelope pressure is maintained at only 5mbar above atmospheric. Air chambers, the so-called “Ballonets” help to keep a constant interior pressure at all flight altitudes, and maintain a rigid hull.

Reading the answers again this one seems valid and better corresponds to the current question title. The other answer is also good but I should probably post second more general question. Probably too general for the stack exchange

I have flown an NT. I believe this is the correct answer.

It's one of the ways it can control altitude. Ballonets work quite well, including in modern airships.
What they do is allow the helium inside the airship to take less than its full volume, by taking up the rest of the airship's volume with air. Buoyancy comes from space where you don't have air, and displacing helium with air reduces that space.
Ascending requires having more helium volume, descending less helium volume. Since the same amount of helium takes more volume at high altitude (less pressure), it works out without a need to vent it.
For an easy analogy, take a submarine. It can float because it's filled with air. Replace some of that air with water - fill the ballast tank - and it sinks. In an airship, outside air acts like water. Water is neutrally buoyant in water, but it reduces the space filled with positively buoyant air.
Think of the ballonets as ballast tanks. The only difference is that helium isn't discarded, since it's difficult to replace.
Expanding the ballonets compresses the helium a bit, but just for a moment, the airship then sinks to where outside pressure is higher, restoring the equilibrium. That small temporary pressure change is easily exerted with blowers.
Alternately, after altitude is changed aerodynamically in forward flight or with propellers, the ballonets need to be expanded or reduced in size to match the new external pressure.
In the picture below, see how the volume of the airship is part helium and part air:

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Strange flight path
I am tracking my husbands plane from Tokyo to Chicago and saw a strange flight path. What would be some reasons for this?

I'm voting to close this question as off-topic because this is a bug in a flight visualization software.

Possible duplicate of Why do aircraft on Flight Radar 24 jump around randomly sometimes?

Closing this as OT seems harsh. The possible dupe that fooot listed is exactly the same question. I guess it's a minor semantic detail whether it's closed as dupe or OT, but I'd have voted dupe.

I don't think that is the actual flight path. Probably a software bug or faulty hardware.

The flight took off 1 hour late and is now running 5 hours behind


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How to keep a helicopter steady when close to the ground?
I understand the ground effect and the air cushion of approximately 3-6 inches when the helicopter is close to the ground. With that in mind, my questions are:

Why does the helicopter become inherently unstable when close to the ground?
What are some methods to keep it smooth when operating close to the ground?

A helicopter is constantly inherently unstable, and requires constant input from a pilot to maintain stability in flight

@CarloFelicione: … except for Kamov-type helicopters. They are inherently stable.

The ground effect can be seen as an increase in air pressure underneath the lifting surface because the down-flowing air cannot stream through the ground. An image from this book::

In the hover, the down streaming airflow is pretty docile, with turbulent whirls only occurring in the transition to forward flight in ground effect. Full scale helicopters are inherently unstable with an eigenfrequency period in the order of a couple of seconds, and the instability gain is not really affected by the ground effect: amount of lift is kept relatively constant by pilot action, and ground effect mainly reveals itself as a reduction in induced drag.
So for full scale helicopters, pitch/roll controllability of the helicopter does not change (much) in ground effect. Depending on objects on the ground and the slope of the terrain there can be changes in trim as the pressure increase influences the horizontal tail for instance, but these do not fundamentally change eigenfrequency period.
For model helicopters, hovering in ground effect is a much more difficult task: the recurring blade period brings on a temporary pressure increase from lift creation. From Prouty:

The wake in ground effect is not really a steady flow, as is assumed by the theory, but a flow with large-scale fluctuations that can be felt as gusts by an observer standing near the helicopter. Reference 1.21 speculates that these fluctuations are associated with the vortex that is made up of the individual vortices from the blade roots. This vortex apparently writhes like a pinned snake along the ground, causing the entire wake to shift and wobble.

Since scale models have a higher revolution than full scale helicopters, the frequency and distribution of these vortices may be what causes higher instability for hovering.

@ koyovis Is there anyway the lessen the effects of this? Such as small circles with the cyclic to create a decreased performance of the blades lift therefore reducing th special effects? Just thinking out loud

@Wes With full scale hell's or with scale models? Have updated the answer, full scale hell's don't seem to have much of a problem with decreased controllability. The moments of inertia are of course a lot higher but I don't know how the effect of turbulence scales with model copters. Smaller rotors have a lower Reynolds number which scales all effects differently.

you are correct in the assumption that I was referring to full scale helicopters I.e the bell 206.


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What is pressure refuelling and how does it work?
In this answer to this question, "pressure refueling" was given as a reason to not worry about fuel vapors escaping when refueling larger aircraft.
What is pressure refueling and how does it work?

With a pressure fueling system, rather than fueling through holes in the tops of the wings, a fuel truck or fuel hydrant is used to push fuel into the airplane through a fitting at about 50 psi. The fuel fitting is generally in the right wing. A fueling control panel allows the fueler to fill the tanks to the amount called out by the flight plan.
These large aircraft still have an overwing fueling port for those times when the pressure fueling system is inoperative.
There is nothing to prevent fuel vapors from escaping to atmosphere.

Is that all it is? Essentially, pumping rather than pouring the fuel? How does this control vapours?

They're not pouring out the open "hole" where the fuel would go in;  they're escaping through dedicated fuel tank vents?

@DavidRicherby the connection is air and liquid tight. There shouldnt be vapor in the first place but if there is it gets pumped into the planes internal plumbing as there is no atmospheric venting possible at the refuel panel.

@casey help enlighten me. How to force vapor into the internal plumbing when you refuel tonnes of JetA at a time? Fuel will occupy space previously taken by vapor. Where does the vapor goes?

@vasin1987 on the way to the tank if there is any vapor in the hoses/plumbing it will be in bubbles suspended within the flowing liquid.  Once fuel gets to the tanks there are vents in the wing to allow air out of the tanks as liquid fuel is pumped in.  Also note the plumbing differs with different airplanes.  Some (like the picture above) pump pretty directly into the tank, while others feature single point refuelling and fuel is delivered to various tanks from a single fuel connection and thus more plumbing is involved in the airplane.

Once fuel gets to the tanks there are vents in the wing to allow air out of the tanks as liquid fuel is pumped in Wouldn't that allow fuel vapor to escape the tank with the air? I understand the system better now, and think it's similar to what they use in V8 Supercars where there's a hose for fuel in, and a separate hose for vapor/excess fuel out like this

There is no hose to remove the fuel vapors on an aircraft, all the vapors vent to atmosphere.

I'm an Air Force fuel systems supervisor.
Yes...air is pushed out of the tank vents during fueling operations, but there are no flammable vapors in jet fuel until the fuel temperature exceeds at least 100 deg. F. There is odor, but not dangerous vapors until that temperature is reached.
The single point system is primarily necessary to reduce spills...it's a closed system that allows faster fueling with less environmental impact.


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Is anti-icing a small area of the windscreen (hot-plates) adequate for the full flight or only an interim solution?
I asked this question about what "the thing on the windscreen of some Piper aircraft is". It came out that it is used as small anti-icing.
The user Jules asked following in a comment of an answer:

Is de-icing this small an area generally beneficial for longer flights, or is it "just good enough" for a landing at the nearest airport if the ice buildup is starting to seem hazardous? (Asking as a non-pilot)

I also think it's a good question, but unfortunately the author of the answer didn't answer.
Thank you!

I also answered that original question. In my answer I attempted to answer Jules's vague question (and your question) in regards to both the practical and regulatory aspect (i.e. the hot-plate has to be sufficient "so that the pilot has adequate view for taxi, takeoff, approach, landing, and to perform any maneuvers within the operating limitations of the airplane."). Is there anything else you wanted clarified?

Possible duplicate of What's that "frame" in the windshield of some piper aircraft for?

I disagree that this is a duplicate. The linked question seems to be asking what the windshield deicer is, and this question seems to be asking whether or not it can be used during normal operations or only as an emergency measure.

Looks like I overlooked it. Sorry for that. But I would still be interested if there are any problems when landing or in general with just a small window to look through.

Those hot plates, in my experience, are useless.  The only reason we had one on our PA34 cargo plane was to make it legal for flight into known icing.  As far as I'm concerned, the whole and entire point of those external hot plates is to check a regulatory box to make the plane legal for FIKI if so certified.
It never once actually kept ice from accumulating and the entire windshield, hot plate included, frosted over on more than one occasion.  I recall hoping for strong right crosswinds so I could crab down to the runway using the side window.  We checked that the circuit was powered so we knew the plate had power, it was just totally ineffective.  Cutting a new pilot loose in that thing for their first low IFR icing encounter was a serious decision.
Now, proper heated windshields with integrated elements work wonders on ice.  They also cost $25k per side (twin Cessnas), so they'd better work.  The plates bonded to the windshields (on Barons, for example) seem to work OK too though I've never used one.  I think the cold air flowing over and under the stand-off hot plates simply overcomes the capabilities of the unit except in the most mild conditions.


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Why is stall speed listed in a POH?
In this comment on this question about stall warning systems, it was stated that:

You can stall at any airspeed (see this question and this one) and in any attitude, only the AoA is important because that's what causes the airflow to separate from the wing.

Why, then, does the Cessna 172's POH (Page ii or 2 of 422) list stall speed, not stall AoA?

I can understand how flying slower than the listed speeds would cause a stall, but the comment above seems to imply that one could stall the plane by flying at 60, 90 or even 120 KCAS but at a high AoA.

Could you please tell me what KCAS is?

@MadhavSudarshan "Calibrated AirSpeed expressed in Knots"

@FreeMan in the questions linked in your quote, see this answer at the point "level flight"

If, for instance, you were flying at 100kts and you suddenly decided to over bank the wings and yank back on the stick you'd likely separate the flow and induce a so called "accelerated stall". You'd still be well above published stall mins, but wouldn't have any laminar flow. Assuming you didn't overstress your aircraft, the fix is simply to push on the stick and unload the aircraft. This instantly reduces AoA and you'd probably find that you were no longer stalling.

So, @federico, in order to maintain level flight at a speed below 53KCAS, flaps up, power off, I'd have to be so nose high that my AoA would automatically lead to a stall?

@FreeMan exactly, yes.

@FreeMan if you were at max gross weight yes. If you are lighter your actual stall speed for that configuration will be slower, so you'd likely be able to do it.

@FreeMan Note that you will not be able to maintain level flight at any speed or AoA with the power off. :) You can fly level with the power off for a while, but you will be losing airspeed rapidly and will eventually stall (presumably near the speeds listed above and at a rather high AoA.)

Those stall speeds are derived for specific conditions.  In the case of that POH those speeds assume level unaccelerated flight, 2550 lbs gross weight, standard atmosphere and no wind.  They are given as a speed rather than raw AoA because most non-aerobatic small airplanes lack AoA indications in the cockpit, while all have airspeed indicators.  A non-aerobatic airplane also spends most of its time in the part of the flight envelope where the assumptions needed to list a stall speed tend to be valid.
The stipulation of level flight means the relative wind is fixed, thus the AoA depends only on the pitch attitude above the horizon.  For an given weight there is a relation of pitch attitude required to maintain level flight for a given airspeed.  Given these specific stipulations, you can determine the airspeed that results in the critical AoA being exceeded.
If any of the criteria listed above are not true, then those published stall speeds are not meaningful to you.  You can stall at any attitude and at any airspeed.
To see the true relation between airspeed and stalling you need to consult a Vg diagram.

The curved lines starting at 0 MPH and 0 load factor represent the stall speed for a given load factor.  The stall speed listed in PoH you quote is specifically the point noted "Normal stall speed" on this chart (this chart isn't for the same airplane, so the numbers will differ).

I think you've restated what Federico linked to in his comment.

@FreeMan So a person goes out of there way to post a detailed answer to a question you asked, and the only thing you can do is complain to them that they didn't click and read through every link in the comment section?  Unreal.

@RhinoDriver, I can see now how it may be taken that way, but that's not the way it was intended. My intent was to get a confirmation of what I said, not to complain about it. Apologies to any who may have been offended.

Cessna 172s don't have an AoA indicator or gauge and even today most light aircraft still don't, although you can add them to some models and the FAA would like more aircraft to have them. So in the absence of an AoA instrument, airspeed is used as a rough proxy, assuming 'normal' flight conditions.
Crudely put, if your C172 is flying at a more or less steady 110kts then the AoA is low and you have a lot of 'reserve' before you stall; if you're flying at 60kts then it's high and you have only a little reserve. This is why watching your airspeed closely on final approach is essential.
But because airspeed is only a proxy for AoA, it isn't a completely accurate way to know how close you are to a stall. Changing the load factor (roughly, the G force on the aircraft) changes the stall speed but not the critical AoA, which always remains the same (18° is commonly mentioned for light aircraft, but that's an approximation). The diagram in @casey's very useful answer shows this clearly.
Finally, note that 14 CFR 23.49 says that light aircraft must stall at a calibrated airspeed of 61kts or less, there's no mention of AoA.

"light aircraft must stall at ... 61kts or less..." Interesting side note to this, larger aircraft DON'T have a single stall speed listed in the flight manual as light aircraft do, because of the effect of large changes in operating weight.  In a light aircraft, the published stall speed typically assumes worst case (i.e. max certified takeoff weight), and other cases (single pilot, no cargo, less fuel) will lower the stall speed slightly. Large cargo aircraft can nearly double their empty weight with cargo+gas, so the range of stall speeds is too broad for 1 value to be useful.

@Pondlife - the actual wording of the regs is single engine airplanes and multiengine airplanes of 6,000 pounds or less maximum weight that cannot meet the minimum rate of climb specified in §23.67(a) (1) with the critical engine inoperative, rather than all light aircraft (MTOW 12,500 lbs. or less). I'm guessing the reasoning behind this is that they want to make sure that the airplane is still controllable at a speed that allows the pilot to deadstick the airplane and have a chance of slowing down to where impacts are generally still survivable by the occupants.

@habu That's correct: I was using the term "light aircraft" in a loose sense, just to make the point that there is a specific regulation related to the question.

@Pondlife - fair enough. just wanted to point it out to avoid any confusion :)

Most small planes do not come with an AoA sensor or display, so giving the stall AoA isn't useful to the pilot.
The bulk of flying is done in the vicinity of level flight, no roll.  When in that configuration, there is a simple relationship between airspeed and angle of attack.  So the airspeed provides useful information about how the wing is likely to be doing.  It must be understood by the pilot when that relationship is no longer valid.

In the (currently) accepted answer to the referenced question is a picture of a Cessna with an AoA sensor/reed. I am not plane spotter enough to know if the pictured plane is a 172, or not, but it is a small Cessna. Therefore, I'm not immediately inclined to accept Most small planes do not come with an AoA sensor without further substantiation. Your answer does, otherwise, make sense.

When I way no "AoA sensor/gauge", I mean a display in the cockpit that shows the current AoA (so you can see how far you are from it).  The stall warning system activates only after you exceed a critical AoA.

The reed is binary.  Either you are exceeding the angle of attack it is calibrated for, or you aren't.  It doesn't show trends or how far from critical AoA you are, both of which are very useful information.  For that, you need a more sophisticated AoA indicator, like this, which tend to only be on more sophistacated aircraft.

But the pictures there don't show an 'AoA sensor' in the sense of an AOA indicator in the cockpit where an actual gauge or meter visually displays the current angle of attack. Those pictures show stall warning indicators, which only sense a particular AoA and provide an audible warning in the cockpit. There's no use for the numeric value of the critical AoA if you don't have an instrument that displays numeric α values.

@BowlOfRed, that makes more sense. You said sensor I thought "the device in the wing", while you meant "an indicator in the cockpit".


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How is the area rule applied on airliners like the A380?
Wikipedia says that because of the area rule's impact on wave drag:

aircraft have to be carefully arranged so that at the location of the wing, the fuselage is narrowed or "waisted", so that the total area doesn't change much.

This much makes sense to me. Though it's a bit difficult to tell, I understand that in the picture supplied there, the green circle toward the front of the aircraft is intended to have roughly the same area as the blue circle and lines through the wing cross-section:
All images from Wikipedia unless otherwise noted
This is more clear in the picture of the F-106 Delta Dart where one can easily see the Coke bottle waist where the wings near their widest:
That's an awesome picture! You could stare at it all day, couldn't you?
Peter Kämpf even provides a nice explanation of the area rule in his answer here including a drawing from the Junkers patent on the subject:
Image posted by Peter in his answer, referenced above
In this image, I can see how the areas through several lines across the fuselage, wings & engines are all intended to be (roughly) equal.

What I don't understand is how this is applied to airliners like the Airbus A380 which they say is:

showing obvious area rule shaping at the wing root

in reference to this image:

To me, it appears that the cross-section at the wing root consists of the same area at the front of the plane (roughly behind the cockpit appears to be where maximum diameter is achieved), plus the area of the wing root, plus the area through the wing, plus the area through a pair of engines (depending on exactly where you make the cross section). As I see it, there's no slimming of the body to make the area through the wing cross section equal to the area of the cross section just behind the cockpit.
This shot from above of another A380 shows that there is no Coke bottle waist near the wings:

Am I not seeing this correctly, or do I not understand the area rule correctly?

I checked your link, but I do not see any mentioning of the area rule. Are you sure the link is correct? Furthermore, I agree with your point, and to me it seems that the area ruling is most obvious from the large pods on the trailing edge, rather than something on the wing root.

@ROIMaison: Which link do you think is incorrect? To me it looks like all the links that I expected to be about area-rule are about area-rule.

@ROIMaison not sure which link you're referring to, but the first one goes directly to the Wiki Area Rule page...

Sorry, I misread, I thought the A380 article would talk about the area ruling of the A380, but it was the area ruling article. Please disregard my comment.

This picture seems to better show some contours aft of the wing.

Interesting, @mins, the wing root pods seem to end rather abruptly, at least from that angle.

You may play with an interactive 3D model on ClaraIO. Example of output.

First of all, not the area should be equal, but the area gradient along the direction of flow should be shallow. The minimum drag with a given volume can be achieved when the area distribution is that of a Sears-Haack body. Ideally this rule applies only at Mach 1, and once you go faster, the cross sections which matter are those along a Mach cone, not those of the aircraft's cross section.
For subsonic flight, the penalty for neglecting the area rule is small; it only matters when local flow is supersonic ahead of a contraction of the contour of the aircraft. Subsonic flow would decelerate, whereas supersonic flow will accelerate further and result in a drag-intensive shock downstream. Adding something to fill up the contraction will reduce the pressure gradients and ideally avoid the shock. By this, area ruling will help to shift the onset of the Mach-related drag rise and enables airliners to cruise a little faster. Honestly, I cannot see the "obvious" area rule shaping on the A380 - to me this is classic subsonic aerodynamics which tries to avoid sharp pressure gradients in the area of the wing airfoil's rear loading. Especially the outer engine pylons could be done better, but I digress.
For airliners, it is much more important to have a constant fuselage cross section, which makes stretching the fuselage simpler and is much easier to build. Area-ruling the fuselage simply is not worth it (yet) when your maximum cruise speed is just Mach 0.85. It is enough to add some Küchemann bodies to smooth out pressure gradients.
Below is a comparison of a swept wing at Mach 0.9, on the left clean and on the right with Küchemann bodies. Note the massive flow separation on the left wing, whereas the flow pattern on the right wing shows attached flow.

Mach 0.9 wing comparison (picture source)

So the 'pods' at the wing root are really only there to smooth the increase in cross section as the leading edge of the wing comes into the cross section? Similar to the wing strakes on an FA-18.

@FreeMan: No, it is about avoiding a steep pressure rise in the rear part on the lower side of the wing. There the airfoil has plenty of camber and is thinned rapidly, in order to gain lift and allow to have high-camber flaps. Without the bump in the fuselage fairing and the flap track fairings, the flow can separate there at low angles of attack. The general concept is to avoid adding two pressure rises, that of the wing and that of the fuselage, at the same lengthwise station.

I think I follow you. However, that doesn't (in my mind), track with what the Wiki article says. Have I misinterpreted the Wiki article, is it wrong, or am I just not putting all the pieces together?

The belly fairing is there to prevent super velocities, it's not related to the area rule

@FreeMan: The Wiki article sounds as if the area rule is the only thing to consider in design. The rear part of the airfoil is where the lowest flow speed can be found, and this is the part that needs area ruling? Sounds odd, right? And bulging fairings right behind the wing root are not exactly "coke-bottling" the fuselage. In truth, both area ruling and decoupling pressure rises are related concepts which lead to the same contours. The Wiki article could be improved if it would also mention this.

For those of us not well versed in the black art of aerodynamics, could you describe a bit more of what we're looking at in the wing pictures? I'd presume the white is smoke so we can see the air flow. The "massive flow separation" you mention in A) is the white line running most of the length of the wing? Is that air rising up (in the Z axis) from the wing, changing direction in the (Y axis) plane of the wing, or something else entirely?

On airliners it is applied via the use of these sharp pods under wings.
A coke-bottle-like fuselage would be extremely impractical for internal arrangement and cargo loading, so it is not done on airliners even though it could probably improve the efficiency a bit.
What is done is:

The engines shifted ahead of the wing help smooth out the increase of cross-sectional area around the wing leading edge.
The enlarged flap track fairings, a.k.a. anti-shock bodies help smooth out the decrease of cross-sectional area around the wing trailing edge.

So the idea is to increase the cross section (or frontal area) as slowly as is reasonably possible (i.e. the front of the engine nacelle is going to happen, and you can't really stretch it fore/aft), not necessarily to keep it constant from front to back?

@FreeMan: Yes. Because that is what the area rule says: that the cross-section should change smoothly.

Here are a couple of oil flow visualization pictures.  Presumably they are taken at the same Mach number and lift coefficient. The left hand picture appears to show the upper surface of the wing with flow separation where the shock meets the surface.  The airfoil sections probably pre-date supercritical designs.  The right hand picture shows the application of Kuchemann carrots (or Whitcomb fairings). The flow appears to be shockless.
The lower picture shows application of these fairings to the Convair 990.  I think I'm right in saying that supercritical airfoil sections, which are designed to weaken the shock, eliminate the benefit of these fairings.
It's not clear to me that the flap track fairings on the A380 (and many other aircraft) have the same (or similar) effect seeing as the flow on the lower surface of the wing is subcritical at just about all operating conditions, as Peter Kampf points out. His analysis implies that the benefit is not due to transonic area ruling, but to other aerodynamic effects, and I'm sure he is correct.


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Why do turbofan blade tips exceed the speed of sound while prop tips should not?
It seems that the tips of the fan of a turbofan engine can and do break the sound barrier:

The GE-90 has a fan diameter of 3124 mm and a rotational speed of 3475 RPM. Their circumferential velocity is d·π·57.917 = 568 m/s or Mach 1.67 at sea level and ISO atmosphere.Peter Kämpf's answer

While, on a propeller driven aircraft, it seems that having the prop tips exceed the speed of sound is a bad thing - Can turboprop blades break the sound barrier?
Why is it that exceeding the speed of sound is acceptable in one situation but not the other?

The Mach 1.67 estimate is too high, because it ignores the fact that the airflow through the fan is constrained by the fan case. The speed difference between the blade tips and the air outside the fan case is irrelevant. The flow through the fan is indeed transonic in some situations, but the highest Mach number is much closer to 1.0 than to 1.67.

@alephzero The blade tips are traveling in a plane that is (mostly) perpendicular to the flow through the engine. It's true that the flow through the engine will usually (probably always in a non-scramjet) be subsonic, but that doesn't mean the blade tips are subsonic relative to the air in which they're traveling, since they're moving perpendicular to said flow. In order for the airspeed of the blade tips to be less than their rotational speed, the intake air would have to be rotating around the turbine axis before encountering the intake fan.

Short answer:

Turbofans need supersonic speed at the fan blade tips to create their high thrust.
Turbofans can tolerate supersonic speeds because the intake creates constant flow conditions irrespective of flight speed.
Efficiency for propellers and fan blades is highest at subsonic flow conditions.
Propellers can turn at supersonic speeds, but since flow conditions are less controlled, the penalty for doing so is much higher than the penalty for a fan.

Explanation
It is a bad thing to have supersonic fan blade tips, just like supersonic propeller tips are best avoided. But in turbofans it is a price worth paying, because the faster tip velocity means higher dynamic pressure, and the pressure difference between both sides of the fan blade grows with the square of their velocity. This makes the high thrust levels of modern turbofans possible.

Propeller efficiency over speed (picture source). The plot for fan blades would look not much different. The very thin, uncambered airfoil of a supersonic propeller and the added wave drag lower the maximum efficiency, but hold efficiency up into supersonic air speeds.
Note that the propeller on the XF-84H Thunderscreech did move at supersonic speed. There is nothing inherent in propellers which prevents their tips from moving faster than the speed of sound. On the other hand, the big diameter of a prop requires proportionally more torque to keep the prop rotating against the drag from the supersonic tips. Thus, a fan engine requires less torque per blade to reach supersonic tip speeds on the fan blades.
Also, the shroud of a turbofan engine helps a lot to make the noise from supersonic tips manageable. The XF-84H's noise made people literally sick. But there is more to it: @FreeMan encouraged me with his comment to dive a little deeper.
A supersonic propeller will work well when the direction of flow at every station along the propeller blade is about equal to the local airfoil chord. Since the blade is uncambered, this means that the change in local flow direction at the leading edge can be minimized to the amount which is required to create the desired thrust. But to be able to fulfill this condition you need to match your propeller speed to the flight speed and twist distribution. Also, the angle of attack must be compensated for by sviveling this propeller axis into the direction of flight. It will have no p-factor, but can run at only one speed for a given flight speed.
Contrast this with a turbofan: The intake makes sure that the flow speed and direction at the face of the fan is the same no matter what the flight speed is. This is done by the pressure field in and around the intake which will spill excess air overboard at high speed or suck extra air in from the sides at low speed. In the fan you can indeed match the local angle of incidence to the airspeed so the fan will work well over its design range.
Generally, a fully subsonic fan would be more efficient. But then the diameter would need to be as large as that of big turboprop engines, and the shroud would become impossibly heavy and produce too much drag. The high dynamic pressure on the fan blades is needed to produce the thrust with the relatively small diameter of a turbofan.

If I read your text and that graph correctly, a very thin, uncambered, supersonic propeller would actually be preferred if one could build a practical shroud/nacelle around it, and that, essentially, is what the "fan" portion of a turbofan engine is.

@FreeMan: There is more to it. This propeller would only work well if the direction of flow at the leading edge is parallel to the airfoil chord, so the local incidence must be right. This requires to match twist, speed and airspeed. For every Mach number you would have only one prop speed where the twist would be right. In subsonic flow the round leading edge gives you much more leeway to be off with your local incidence. The intake of a turbofan gives you that: Uniform and equal airspeed no matter what flight speed. A propeller needs to be more flexible.

@FreeMan: Yes, the fan is really a propeller with easier flow conditions and a very high activity factor.

@Thesis: The question is about the blade tips, so I refer to the speed at the blade tips here. The intake flow speed and the flight speed are indeed subsonic, but the circumferential speed of the blades must be added. I will edit to answer for clarification.

@Thesis: (1) Yes, but strictly true only for the single fan blade. Since the solidity of a fan is higher than that of a propeller, the torque to drive the whole fan is immense. But the torque/thrust ratio of a fan is lower. (2) A subsonic fan would run much slower and have less dynamic pressure along the blade span. Not D but RPM needs to be small. To produce thrust, the subsonic fan would need more air, which it gets by increasing D. Thus, it becomes a propeller.

Does this mean that it actually is practical to make a supersonic prop aircraft, so long as you have a shroud, the engine can be gimbaled, and the propeller blades have variable pitch AND twist? Also, I don't understand why, if turbofans are shrouded and I think meet your conditions, they are basically never used for supersonic flight. Thanks a lot.

@Gus: Supersonic flight needs high exit speeds of the exhaust gasses, so a turbofan is at a disadvantage. The Tu-144 used turbofans first and switched over to turbojets (which the Concorde used from the start) with considerable performance improvements. See here for more. The shrouded supersonic prop is indeed feasible, but much easier to achieve when coupled to a jet engine. Most supersonic aircraft today use turbofans with a low bypass ratio.

@PeterKämpf Thanks. I'm wondering now if the shrouded supersonic prop would be practical for a supersonic electric aircraft? The issue I see is the mass of the motor and duct needing to be gimbaled. The Concorde's engine, Olympus 593, has a power to mass ratio of 36 kW/kg, while high performance electric motors only have 9 kW/kg, which means the assembly would be heavier than that with a turbo instead (but maybe it's smaller in volume). However, relative to other options for supersonic electric aircraft (compressor+nozzle combo) perhaps it's optimal? Also, any reading suggestions for this?

Maybe some sort of contra-rotating scimitar blade could be used, as well.

@Gus: Forget electric propulsion for supersonic flight. This is utter nonsense with current (and projected) energy storage and realistic range. Also, the 593 power is calculated at Mach 2 and includes intake thrust; you will be surprised how much this will affect the calculation. Look at static performance instead for a fair comparison.

I was thinking an advantage propellers have is relatively constant efficiency with altitude. So you could go really high (85,000 ft) and have about 1/3rd of the drag of a Concorde assuming drag is ~proportional to rho (not sure w/ wave drag though). And if it's possible to design a supersonic prop like the one you described, near the efficiency of low speed props, then perhaps the engine can be 1.5-2x as efficient as the 30-40% of jets. And so then the cruise power requirement would be cut to ~25% of the Concorde, then maybe it makes sense if batteries double in J/kg from their current state?

@Gus: Electric propulsion has the advantage of high efficiency over a high motor speed range, so the high altitude propeller must be very large (to compensate for the low $\rho$ at altitude) and slowly spinning at low altitude. But with an intake-like shroud for supersonic efficiency the propeller needs to become much smaller and weighs a lot (and still efficiency will be lousy compared to subsonic efficiency). Look here for subsonic electric flight.

Propeller tips can and sometimes do go supersonic (like XF-84H or Tu-95). However, while it is tolerated in case of turbofan (some measures like swept blades and low speed fans are used in turbofans to counter this), it is not so in case of propellers for a few reasons:

As the supersonic speeds are approached (or exceeded locally), shock waves form over sections of the propeller blades- This significantly reduces the propeller efficiency while at the same time causes increased loads on the blade. This causes a problem- For supersonic speeds, the blades have to be extremely thin, while the loads require the blades to be thicker.
The drag due to the (tip) shock waves increases the required engine power tremendously. For example, the Tu-95's engines had to be uprated from 12000 shp in the prototypes to 15000 in production units to reach the required speed.
Another main reason is sound- XF-45 was so loud that it caused seizures. In case of turbofans, the bypass fan is covered, which mitigates the noise issues somewhat.

I thought the Tu-95 question established that the blade tips do not in fact reach Mach 1.0?

@egid I think that question establishes that the propeller tips went supersonic after the engine power was increased. See the edit in that question.

Also, Peter just edited his Tu-95 answer to correct the prop speed.

@FreeMan ah, yes, indeed. Hadn't seen that - it was M0.87 or something before

The most important difference is that the fan blades are running inside a casing, and propellers operate in free air. Most of the noise and energy loss comes from the vortices shed from the tips of the prop blades. The fan case prevents those vortices from forming, except for the small amount of air leakage between the blade tips and the casing.
There is inevitably a small clearance gap between blades and casing, for example because the external aerodynamic forces on the casing may deform it into non-circular shape in some flight conditions, but the maximum gap around a 3000mm diameter fan would typically be less than 5mm.
Ducted propellers are used in small sizes, but the weight penalty of a large duct that is strong enough to survive conditions like birdstrike (and the collateral damage from bent or broken prop blades) would be prohibitive for large props.

"The fan case prevents those vortices from forming": This seems to be the best and most direct answer to the question.

Why is it that exceeding the speed of sound is acceptable in one situation but not the other?
Because lift (or thrust of the propeller which is a wing that rotates) at supersonic speeds decreases while compression is not influenced. The air that strikes the blades of a compressor has no other way to go but to pass through it and feed the engine.
In the case of a propeller at supersonic speeds the air instead of flowing around the blades forms V waves, a phenomenon that increases drag and reduces lift (thrust).

Do you have some link where one could find more about the V waves (searches tend to return the medical version, which is clearly irrelevant here)?

Instead of "V waves" search instead for "Shock waves".

Oh, shock waves are obvious. I hoped that I'll learn something new, but apparently not.


text:
Why does condensation form on the wing especially during take-off and landing?
To start off, yes this question was asked before, yet I don't find the answers satisfying and here is why:
It appears to me that this phenomenon only appears during the approach phase of a flight and during low speed turns, or rather when the angle of attack is rather high. Also, usually the flaps are deployed when it happens, yet I'm not sure if that makes a difference.
Considering this, the answers wouldn't necessarily answer my question. They all talk about how there is a lower pressure above the wing, which causes the dewpoint to drop and thus makes the water condense. But is there a "lower" lower pressure when the angle of attack is higher and therefore making this phenomenon occur more often? Or is the point at which the laminar airflow gets turbulent (which moves forward as the aoa is increased) an important factor here?
Here is a picture to show what condensation I mean:

And a more extreme one:

Yes, speed and flaps do make a difference.
The lift created by the wings is approximately constant over the duration of a flight. With full tanks, it is higher at the beginning of the flight, and when the aircraft banks, it is again higher to create the centripetal force for changing its course. But the difference is small¹.
Lift is caused by a pressure difference between the two sides of the wing. In cruise, this difference is low but extends over much of the wing's chord. At low speed, however, the suction on the top surface has a peak right aft of the leading edge, so the maximum suction is bigger than it is in cruise. Exactly this changed pressure distribution causes the humidity to condense, because it lets the air's relative humidity rise above the condensation point. In cruise, the pressure change is lower and the dew point is not exceeded. Also, since the airplane flies slower when close to the ground, the air has more time for the condensation to develop.
See below for a color-coded plot of the speed distribution over a typical airliner airfoil in the landing configuration. Red symbolizes the lowest speed (highest pressure), while purple stands for the highest speeds with the lowest pressure. Such suction peaks must be avoided in cruise to enable the aircraft to fly at a high cruise Mach number.

Airliner airfoil with high lift devices in landing configuration (picture source)
¹ The fuel fraction of airliners rarely exceeds 40%, and the added lift in a 30° bank is only 15%.

There are two more reasons for why this happens mostly during take-off and landing: 1: You can most often see it just below the clouds, or close to the ground, in some places which also often have fog because that's where humidity is highest.  2: The required lift during approach is not higher than in cruise, so the absolute pressure difference isn't, either -- however, the pressure distribution is less even. For example, you'll often see water condensing in the very strong flap tip vortices (which are closed in cruise, of course).

As the angle of attack increases, the pressure on the upper surface of the wing becomes lower. The following figure shows this. Note that the pressure on top is suction.

Source: avstop.com
During the landing phase, the angle of attack of the aircraft is higher compared to to other regimes. This is the reason condensation clouds are formed during the landing phase.
However, the clouds are formed during any subsonic maneuver that reduced pressure over the upper surface of the wings, like high-g turns.

Source:www.telegraph.co.uk

Moisture is more prevalent in the lower troposphere, which contributes to seeing the phenomena more commonly at low altitudes.

+1 great picture

Let's remember for a second that on the upper side of the wings there is lower pressure than the free stream (air far from the aircraft).
If the aircraft is travelling through humid air, the adiabatic process (which all packets of air that end up near the wing have to "suffer") dictates that:
$$P_{\infty} ^{(1-\gamma)} \cdot T_{\infty} ^\gamma = P_{wing} ^{(1-\gamma)} \cdot T_{wing} ^\gamma $$
and since we just remembered that $P_{wing} < P_{\infty}$, it means that $T_{wing} < T_{\infty}$.
Still from the wikipedia link above:

Adiabatic cooling occurs when the pressure on an adiabatically isolated system is decreased, allowing it to expand, thus causing it to do work on its surroundings. When the pressure applied on a parcel of air is reduced, the air in the parcel is allowed to expand; as the volume increases, the temperature falls as internal energy decreases.

If the initial $T_{\infty}$ was already low enough, or if there was enough umidity in the air, the packets of air above the wings finds themselves below the dew point, producing the condensation that prompted your question.

But is there a "lower" lower pressure when the angle of attack is higher and therefore making this phenomenon occur more often?

Yes, higher angles of attack mean higher lift, that is generated by higher differential pressure between upside and underside of the wing. Remember that the lift is roughly
$$L \propto \alpha \cdot V^2$$
and since the weight of the aircraft is not reduced, to keep the lift constant at lower speeds (such as the speeds just before landing) you need an higher $\alpha$.
To prevent the need of extreme angle of attacks, the effect of the flaps is to increase the $C_{l\alpha}$, meaning that for the same angle of attack you get more lift, changing the proportional relationship above.

When you next snap open a can of fizzy drink watch carefully at what first comes out of the hole.  A tiny cloud of condensation that quickly disperses.  This is your personal adiabatic decompression demonstrator.  It's exactly the same Physics as what's going on in the photos above.

it appears to me that this phenomenon only appears during the approach phase of a flight.

Flying at high altitudes where it is -40°F fuel in the wings cold soaks.
That is my answer for the first two pics of the commercial aircraft above, the one pic of the vapor bubble or whatever about the fighter jet I would remove from the discussion too many unknowns about the picture regarding speed and the maneuver.  Commercial Jet aircraft flying > 30,000 feet altitude for X amount of time the wing becomes a nice cold plate increasing the likelihood and making it more common to see during the approach/landing phase of a commercial flight.

It's unrelated to the temperature of the wing. The cause of condensation is temperature drop due to adiabatic expansion of air over the wing.

if it had nothing to do with temperature of the wing/fuselage then "phenomenon" would be seen at same amount during climb to altitude as it does on descent.  It does not.  Temperature of the fuselage is a driving factor in addition to the air temp and humidity on descent.  water vaporizes easier under lower pressure, approach of 200+ knots = vapor cloud under appropriate conditions: colder aircraft or more humidity, or greater pressure drop about the wing (flaps) will increase the likelihood of the vapor cloud.

The lift coefficient during climb to altitude is lower that during final approach, that is why you see more condensation during the approach / landing phase, but not exclusively. The temperature has no effect; on approach, the airspeed is approximately 90 m/s. The wing chord is approximately 4 meters. The exposure time of the air to the wing is thus 44ms; way to short for any significant heat transfer. Also note that the condensation is not only at the wing surface, but also further away; another pointer that it is not due to heat transfer from the wing.

I think you forgot to mention the speed of N1 and N2.  I'm out.

I have a video of a 777 taking off out of ORD on a 70Deg F day and there’s plenty of condensation above the wing. As mentioned above temperatures of wing surface due to fuel would have no effect in this case. It would be a result of pressure differences.

what was the turn around time of the plane was and where did it come from?

classic internet.  someone explain fog and let me read how its formed by pressure differential and not at all by temperature...

Vapor  becomes visible when an air mass is cooled below its dew point. How it is cooled can vary but the end effect will be the same. With the airplane wing it cools due to adiabatic expansion of the air due to pressure change.
Fog can form the same way when the pressure drops, or when the air mass is cooled by radiation at night, by coming into contact with a colder mass of air, by being pushed up to a higher altitude due to terrain (lenticular clouds), by vapor from a warm body of water contacting cold air above the water, etc.

every time i see the same condensation cloud over the river near home, I wonder what the air pressure difference is between me on the road couple hundred yards away where there is no condensation versus the air pressure over the water where the opaque condensation cloud is.


text:
Why is it advantageous to vary sweep angle with relative thickness?
In the answer to this question, Peter Kampf stated that:
"In the majority of cases it is aerodynamically advantageous to vary sweep with relative thickness, but it is not structurally and economically efficient."
Why is this the case?

Here's a high speed reason:
TL;DR: They counter each other's effect on the wave drag
Thickness versus wave drag
Basically, a thicker airfoil has a bigger curvature, and will accelerate the airflow around it more. This leads to more lift for the same speed. However, at cruise speed, closer to M=1, sonic effects will occur. This will lead to increased drag.
This can seen from the figure below, with increasing $t/c$, the value of $c_d$ is higher, and the rapid increase associated to wave drag starts earlier. The value of $M$ at which this occurs is called the Drag Divergence Mach number, $M_{DD}$.

Source: NASA supercritical airfoils: A matrix of family-related airfoils
Sweep versus wave drag
When a sweep angle is present, we can decompose the velocity in two components, of which only the one perpendicular to the chordline ($V_\bot$) is effecting lift (and thus wave drag).
By increasing the sweep, we can reduce the value of $V_\bot$, allowing the airfoil to experience a lower value of $M$, thus delaying the drag rise.

Source: Wing Shape Multidisciplinary Design
Optimization
Combined effects
As you can see, for wave drag, the two counteract eachother. If you have a certain value of $t/c$ and a certain $\Lambda$, this will lead to a certain $M_{DD}$. If for some reason, you want to increase the value of $t/c$, the wave drag increase will come earlier, leading to a lower $M_{DD}$. To maintain the same $M_{DD}$, you could counteract the effects of a larger $t/c$ by increasing $\Lambda$.
This is clearly shown by the emperic Korn-Lock-Mason method, which states:
$$ M_{DD} = \frac{K_A}{cos \Lambda} - \frac{t/c}{cos^2 \Lambda} -\frac{c_l}{cos ^3\Lambda} $$
This also shows the effects of lift on the value of $M_{DD}$.


text:
When do pilots actually use ‘wilco’?
I know it means “I have understood the instruction and will comply.” However, any instruction that

is a clearance,
concerns runway use or
contains altitude/level, heading, speed or squawk code

should be read back. And that seems to cover most instructions.
So what is an instruction, but does not require a read-back and may be instead answered by just ‘wilco’ (+ callsign)?

Allowable use of wilco will vary by jurisdiction.

When instructed to report something.

ATC: Cimber 626 turn left heading 120, cleared ILS approach runway 09, report established
Pilot: Heading left 120, cleared ILS approach runway 09, WILCO, Cimber 626

ATC: OYABC airborne time 33, report passing Dalhem
Pilot; (Roger), WILCO, OYABC

ATC: Lufthansa 135, traffic is a Boeing 737 on 5 miles final for runway 09, report in sight
Pilot: WILCO, Lufthansa 135

The only one of these that I would use is the last one. On the first one, you’ve already read back the required parts of the instruction and Wilco just confuses things. On the second one, you don't need both Roger and WIlco. however, unless I was in really busy airspace, I would read back the reporting point.

@JScarry The example I've used are in compliance with the ICAO Manual of Radiotelephony (Doc 9432). I'm not saying this is exactly how pilots are doing it - but it's how they should be doing it.

Off the top of my head, the only times that I use 'wilco' are when I'm issued 'report established' or 'report airport in sight'. I don't even think @JHougaard's example of 'report passing Dalhem' is appropriate. If a fix is included in the instruction, you need to repeat the name of the fix.  Even if that's not required by regs, I'm repeating it so that I know that I got the name of the fix correct.  I think it's better to be absolutely clear that you're on the same page, even if the mic is keyed for 2 seconds longer.

According to the Pilot Controller Glossary:

WILCO− I have received your message, understand it, and will comply
with it.
ROGER− I have received all of your last transmission. It should not be
used to answer a question requiring a yes or a no answer.

If you are a general aviation pilot, ATC wants to be sure that you have understood the instruction that you have been given and will expeditiously comply with it. If you are given a heading for traffic, altitude, clearance to land, hold short instruction, or clearance into Class B airspace, you should always repeat it back. I would not mix and match Roger and Wilco in readbacks.
Some examples:
At my home airport when given a taxi instruction:

Tower: 90J Taxi to parking via Echo, Juliet, Mike. Remain this frequency.

I could reply Wilco, and my abbreviated callsign. e.g.

Wilco, 90J

At an unfamiliar airport, unless the taxi instructions were very simple, I’d read them back.
When ATC can immediately see that I have complied with the instruction and it is fairly lengthy, I could use Wilco.

Tower: 90J Make a 360 to the right for spacing, watch for birds on final, follow the Cessna on left base.
Tower: 90J Extend your downwind. I’ll call your base.

On the other hand, if it involves a safety of flight issue, I’ll read back what I am going to do.

Tower: 90J Follow the Cessna entering left base.
90J: Looking for the Cessna 90J.

I don’t know that I have ever used Roger. It is usually simpler to just repeat the information or respond with what you are going to do.

ATC: 90J traffic 12 o'clock 7 miles north bound 3,500'

I could respond with Roger, but I want them to know that I ether have the traffic in sight or that I am looking for it. So I would respond with either "Traffic in sight" or "Looking".

ATC: 90J Santa Barbara altimeter 29.92

I suppose you could respond with Roger, but repeating the setting is the preferred response.
I have heard the tower and flight following use Roger—although it’s often “Roger that”.
I am in the practice area and then let ATC know that I am going to fly somewhere else.
Examples:
When I’m starting my VFR descent to the airport.
Reporting birds near the airport.
If you listen to Live ATC, you will hear airline pilots use Wilco in busy airspace, but in my experience as a general aviation pilot, it is quite rare.

As a controller, I would very much like to stress that you should not use Wilco in lieu of a readback when a readback is required. Taxi instructions or instructions to fly a certain procedures (e.g. a 360) must absolutely be read back. Every rule book says so. This applies even on a busy frequency. We will not issue instructions to you unless we have time to listen to the readback. Replying with Wilco will only add doubt as to whether you have actually understood the instructions given, and could easily result in extra workload for the controller.

@JHougaard In my experience, the only taxi instructions that controllers insist on hearing read back are 'hold short of runway XX'. And that is relatively recent, maybe 5 years or so..  I’ve been re-reading the AIM and in 4−3−18. Taxiing it states “1. Good operating practice dictates that pilots acknowledge all runway crossing, hold short, or takeoff clearances unless there is some misunderstanding, at which time the pilot should query the controller until the clearance is understood.”  It does not explicitly state that you must read back the taxi instruction. Do you have another reference?

@JScarry J. Hougaard is a controller in Europe; standards are often slightly different there.

In some situations I'll acknowledge a controller by just replying my callsign, like when they are telling me that new weather information is available. Obviously this is VFR only. At some extremely busy airports/times we also reply just by keying our mic.

Replying 'wilco' for taxiway instructions sounds like a great way to frustrate a ground controller at a busy airport with lots of active taxiways.  Off the top of my head, the only times that I use 'wilco' are when I'm issued 'report established' or 'report airport in sight'.  I don't even think @JHougaard's example of 'report passing Dalhem' is appropriate.  If a fix is included in the instruction, you need to repeat the name of the fix.

@Dustin It depends on the situation. The Fedex pilot who takes the same route every morning doesn’t need to repeat the taxi instructions since they both know what he’s going to do. I have a couple of different routes from the runway to my hangar and if there is no one else moving on that side of the field, they will often give me the option of either one. There’s no point in telling them how I am going to taxi, since they really don’t care. If I respond 'Wilco', or more often 'Taxi to parking', they can get back to handling traffic.